Acoustic studio with variable reverberation time



Feb. 20, 1951 GRAHAM 2,542,663

ACOUSTIC STUDIO WITH VARIABLE REVERBERATION TIME Filed April 30, 1948 W I I I 11111 1/ II I a 6 0 6 I I AREA OF COMPENSATION l ASSIGNED 70 A I ACTUAT/NG DEV/CE PARTICULAR maA s- AMPLIFIER 3 I i M/CROPHONE NT ACTUAT/NG GE RGE K. GRAHAM ATTORNEY Patented Feb. 20, 1951 ACOUSTIC STUDIO WITH VARIABLE REVERBERATION TIME George K. Graham, Oceanside, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1948, Serial No. 24,203

10 Claims.

This invention relates to a new and improved studio arrangement having an effectively variable reverberation time.

An object of this invention is to provide a novel studio arrangement with variable reverberation time, havin means to vary the reverberation time in all or selected areas throughout the studio.

In broadcast studios, it has been the practice to achieve the desired reverberation time by means of acoustical absorbent treatment applied to the interior surface of the studio. With such treatment, the reverberation time is decreased as the sound absorption of the acoustic treatment is increased. Most practical acoustic treatments result in a reverberation time which v varies with frequency. Generally speaking, the absorption characteristics decrease as the frequency is lowered and remain substantially constant for a'band of frequencies in the central portion of the audible spectrum; and the absorption efiiciency again decreases as the higher frequencies are approached. This has the effect of producing a reverberation time which is different for each of the discrete frequencies in the audible spectrum. Experience has indicated that such variable reverberation time, if confined to prescribed limits, results in an improvement in the acoustics of the studio. Ideally, there would be an optimum reverberation time for each type of program presentation. As a result, present day studios are designed somewhat on a compromisebasis to provide a reverberation time that satisfies the majority of re- This invention has means which quirements. compensates for the various factors of the studio reverberation period to give the most desirable or optimum reverberation time characteristic for any program requirement. Reverberation time for the purpose of this invention may be defined as the length of time for sound energy in an. enclosed space which has reached steady state con ditions to decay to 60 decibels.

This invention will best be understood by referring to the accompanying drawings, wherein: Fig. 1 is a plan view showing the arrangement of a studio, setting forth the principle of this invention;

Fig. 2 is a section of one wall and a floor plan of the studio arrangement of this invention; and Fig. 3 is a schematic circuit diagram of the variable system of this invention.

Referring now to the showing in Fig. 1, a source of sound initiating from the platform is indicated as S. At point Pl, located a distance sound source S to the microphone MI.

spaced the distance D, DI, D2 and D3.

D from the sound source S, there will be a time difference between the sound at the source and the point P. For all other points in the room located an equal distance from S, the time difierence will be substantially the same. Similarly, for points P2 having a distance DI from the sound source S, all points in the room lying an equal distance from S will have an equal time difference. According to my invention, if a studio were provided with ducts N and NI together with an acoustic treatment having an absorption in excess of the value required to produce the minimum optimum reverberation time, and the absorption properties of such acoustic treatment were effectively reduced by the addition of sound energy, the reverberation time would consequently increase.

Fig. 2 illustrates one side wall and a floor plan of a studio which is provided with an acousticduct N which is large relative to the area of the studio. Duct N has an acoustic efiect of tending to make the studio larger. A similar duct NI may be placed on the other side wall. The duct N, runs parallel to the Wall and has placed within it a series of microphones Ml, M2, M3 and M4 Microphone Ml will receive sound from the initiating sound source S at a time difference dependent upon the distance from the microphone MI to the sound source S. As previously indicated, all points in the room an equal distance from the sound source will receive sound at the same time. The microphones are preferably located in the acoustic ducts in the walls or the ceiling; or if desired the microphones may be hung down from the ceiling by any suitable means such as support wires, at selective points which may or may not be equidistant from the sound source S. The selected distance could be considered as equal to a spherical radius distance R from the If, for example, a microphone MI is connected to the input of an amplifier whose output is connected to a system of electrical transducers or loudspeakers Ll, L2, L3, L4, eta, located about the room (as mentioned above), then the sound output from the system of loudspeakers so connected will have the same time relationship to the sound source S as the sound reaching microphone Ml. By suitable switching and variable attenuation adjustment of the power output of the amplifier, according to this invention it will be possible to replace all or a portion of the sound energy lost by the absorption of the acoustic treatment material and in such a manner as to effectively reduce the absorption. It is desirable that the frequency characteristic of the microphoneamplifier-speaker system be compensated in such a manner as to provide the desired reverberation frequency characteristic with variable frequency compensation means associated with the amplifier it will be possible to adjust the reverberation characteristic of the studio to meet a wide variety of requirements. Placed within the duct N, at the distance DI, D2, D3, etc. are additional microphones M2, M3, etc. which are also connected to similar separate amplifiers as is microphone Ml. Each microphone amplifier combination is arranged to feed a system of transducers or speakers located at points in the room having the time difference between them equal to time diiferences at the initiating microphone and the original sound source S.

The schematic circuit diagram of Fig. 3 shows the means of varying the volume of sound within predetermined areas.

By suitable switching and volume adjustment of the amplification or energy compensation which is accomplished by means of an attenuator A connected to the amplifier, the amount of sound returned to selected areas in the room (change in net absorption) can be varied at will before and during a broadcast program by the manual operation of a compensation device, and the reverberation time of such a studio may be controlled between the limits set by the maximum absorption of the acoustical material and the maximum permissible sound energy returned to the room or an enclosed chamber such as for example an echo chamber. With such a system, in addition to the effect of changing the reverberation time of the studio by means of the microphone-amplifier-speaker combination, persons seated in the studio will hear sound directly from the source S and from the loudspeakers placed about the room, and hear both sources of sound in the proper time relationship, so that such a system would provide some measure of sound reinforcement in addition to its usefulness in changing reverberation time.

In the above mentioned description, the micro-- phones are located throughout the acoustic duct and the loudspeakers are placed so that their sound output is returned to the room at the same time difference as is the received sound by each microphone. A variation would be to connect a system of loudspeakers located at a point having a time difference greater or less than the time difference from the sound source to the microphone. In this way, it will introduce sound energy having a time relation that is different from the energy reaching the microphone. This would have the effect of providing additional sound paths and increasing the number of time difference reflections which combine to produce the reverberation time effects.

' The placement of a microphonewithin the acoustic duct and careful shaping of the duct opening will reduce or entirely eliminate any sound coupling between the loudspeaker and their respective microphones. In the intended purpose of such microphone-amplifier-speaker combination, the amount of sound energy returned to the room by the loudspeakers will be proportionately less than that received by the microphone.

The total energy available to operate the microphone will be determined by the area of the microphone actuating element. The amount of sound returned by the transducers or speakers will have a value that can be related to a fixed wall surface area. By properly proportioning the sound output, the reverberation period may be changed to suit conditions. It may be desirable to use separate loudspeaker elements for the low and high frequency sound energy and in this event pairs of low and high frequency speakers can be arranged about the room in the manner described above. The number of such speakermicrophone-amplifier combinations may be held to a minimum by careful placement of the units throughout the enclosure.

What is claimed is:

1. A variable acoustic system comprising a chamber in which sound is transmitted, said chamber having acoustic absorption treatment in excess of the value required to produce a minimum optimum reverberation time, a duct in said chamber and having an opening facing said chamber, a microphone located within said duct and substantially registering with said opening, a transducer selectively located within said chamber, means to connect said transducer to said microphone, compensation means, said compensation means being interposed between said microphone and said transducer to effectively reduce said absorption treatment therein to increase the reverberation period within said chamher to an optimum value.

2. A variable acoustic system comprising a chamber in which sound is transmitted, said chamber having acoustic absorption treatment in excess of the value required to produce a minimum optimum reverberation time, a duct in said chamber and having an opening facing said chamber, a microphone selectively located within said duct and substantially registering with said opening, a transducer selectively located within said chamber, an amplifier, means to connect said microphone to said amplifier and means to connect said amplifier to said transducer, said amplifier having variable frequency compensation means to adjust the reverberation time characteristics of said chamber.

3. An acoustic system having a variable reverberation time, comprising wall members defining an enclosed area, an initiating sound source located in said area, a duct in said enclosed area and having a plurality of openings facing said enclosed area, a plurality of microphones ar ranged within said ducts at said openings to re ceive sound, an amplifier system, means to corn nect said amplifier system to said microphones, a plurality of transducers, means to connect said amplifier system to said transducers, said transducers being located at distances from the initiating source of sound equal to the distance between the initiating source of sound and the initiating microphone from which said source of sound originates with respect to the associated trans ducer, and compensating means, said ccmpensat ing means being connected to said amplifier system effectively to vary the energy output of said transducers.

4. A studio arrangement having means for varying the reverberation period comprising a studio having a plurality of walls, an initiating microphone within said studio, ducts in at least two walls of said studio, each of said ducts having an opening facing said studio, a series of microphones selectively located within said ducts and registering with the openings thereof, a plurality of amplifiers, each one of said microphones being connected to one of said amplifiers, a plurality of transducers located within said studio, means to connect a number of said transducers to each amplifier, the distance from the initiating microphones and each of the transducers connected to any one amplifier being equal to the distance between said initiating microphone and the "one of said series of microphones connected to the amplifier under consideration, and means to vary the energy output of each amplifier.

5. An acoustic compensation arrangement having means for varying the reverberation time comprising a plurality of walls defining an echo chamber, a duct located in some of the walls or said echo chamber and having an opening facing said chamber, a source of sound located within said chamber, a series of microphones arranged within the ducts and registering with said opening, a plurality of amplifiers, each one of said microphones being connected to an individual amplifier, and a plurality of transducers, said transducers being connected to said amplifiers, the transducers connected to any one amplifier being located at the same distance from said source of sound, said distance being equal to that from the initiating microphone connected to said one amplifier and said source of sound.

6. A studio arrangement having means for varying the reverberation period, including wall portions defining an enclosed chamber, said wall portions having acoustic absorption treatment in excess of the amount required to produce a minimum desirable reverberation period, at least one of said wall portions having a duct therein opening into said enclosed chamber, a source of sound located at a point within said enclosed chamber, a microphone located in said duct and in registry with said source of sound, an amplifier, a pluralit of transducers, means to connect said microphone to the input of said amplifier and to connect said transducers to the output thereof, said transducers each being located at a distance from said sound source equal to the distance between said sound source and said microphone, and means to vary the gain and frequency response of said amplifier whereby the reverberation period of said chamber is varied.

7. An acoustic system according to claim 3, wherein the microphones are arranged throughout selected areas of the enclosed area where sound is transmitted.

8. A studio with variable reverberation time comprising walls defining an area having an excess amount of acoustic absorbent treatment to produce a minimum optimum reverberation time, a duct located in the walls of said studio and having an opening facing said studio, an initiating sound source located within said studio, a plurality of microphones, some of said microphones located within said duct and registering with said opening, an amplifier system, means to connect said microphones to said amplifier system, a plurality of transducers, said transducers being electrically connected with said amplifier system, said transducers being located at a distance from the initiating source of sound equal to that between said source of sound and the microphone from which said source of sound originates, and means to selectively change the output volume of said transducers.

9. A studio with variable reverberation time comprising walls defining an area having an excess amount of acoustic absorbent treatment to produce a minimum optimum reverberation time, ducts located in two walls of said studio, each of said ducts having an opening facing said studio, an initiating sound source located within aid studio, a plurality of microphones in said ducts and registering with said openings to receive sound, an amplifier system, means to connect said microphones to said amplifier system, a plurality of transducers, means to connect said transducers electrically to said amplifier system, said transducers being located at a distance from the initiating source of sound equal to that between said source of sound and the microphone from which said source of sound originates, and switching means to selectively actuate certain selected transducers within said studio.

10. A studio arrangement having means for varying the reverberation period, including wall portions defining an enclosed chamber, said wall portions having acoustic absorption treatment in excess of the amount required to produce a minimum desirable reverberation period, a plurality of said wall portions having ducts therein, opening into said enclosed chamber, a source of sound located at a point within said enclosed chamber, a plurality of microphones located in said ducts and in registry with said source of sound, a number of amplifiers, a plurality of transducers, means to connect said microphones to the inputs of said amplifiers and to connect said transducers to the outputs thereof, the transducers connected to a given amplifier each being located at a distance from said sound source equal to the distance between said sound source and the microphone connected to the input of the amplifier under consideration, and means to vary the gain and frequency response of said amplifiers whereby the reverberation period of said chamber is varied.

GEORGE K. GRAHAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

